Preparation, characterization, and immunogenicity of Haemophilus influenzae type b polysaccharide-protein conjugates

Antigen processing of glycoconjugate vaccines; the polysaccharide portion of the pneumococcal CRM(197) conjugate vaccine co-localizes with MHC II on the antigen processing cell surface

Pneumococcal (Pn) polysaccharides (PS) are T-independent (TI) antigens and do not induce immunological memory or antibodies in infants. Conjugation of PnPS to the carrier protein CRM(197) induces PS-specific antibody in infants, and memory similar to T-dependent (Td) antigens. Conjugates have improved immunogenicity via antigen processing and presentation of carrier protein with MHC II and recruitment of T cell help, but the fate of the PS attached to the carrier is unknown. To determine the location of the PS component of PnPS-CRM(197) in the APC, we separately labeled PS and protein and tracked their location. The PS of types 14-CRM(197) and 19F-CRM(197) was specifically labeled by Alexa Fluor 594 hydrazide (red). The CRM(197) was separately labeled red in a reaction that did not label PS. Labeled antigens were incubated with APC which were fixed, permeabilized and incubated with anti-MHC II antibody labeled green by Alexa Fluor 488, followed by confocal microscopy. Labeled CRM(197) was presented on APC surface and co-localized with MHC II (yellow). Labeled unconjugated 14 or 19F PS did not go to the APC surface, but PS labeled 14-CRM(197) and 19F-CRM(197) was internalized and co-localized with MHC II. Monoclonal antibody to type 14 PS bound to intracellular type 14 PS and PS-CRM(197). Brefeldin A and chloroquine blocked both CRM(197) and PS labeled 14-CRM(197) and 19F-CRM(197) from co-localizing with MHC II. These data suggest that the PS component of the CRM(197) glycoconjugate enters the endosome, travels with CRM(197) peptides to the APC surface and co-localizes with MHC II.

How B cells shape the immune response against Mycobacterium tuberculosis

Extensive work illustrating the importance of cellular immune mechanisms for protection against Mycobacterium tuberculosis has largely relegated B-cell biology to an afterthought within the tuberculosis (TB) field. However, recent studies have illustrated that B lymphocytes, through a variety of interactions with the cellular immune response, play previously underappreciated roles in shaping host defense against non-viral intracellular pathogens, including M. tuberculosis. Work in our laboratory has recently shown that, by considering these lymphocytes more broadly within their variety of interactions with cellular immunity, B cells have a significant impact on the outcome of airborne challenge with M. tuberculosis as well as the resultant inflammatory response. In this review, we advocate for a revised view of TB immunology in which roles of cellular and humoral immunity are not mutually exclusive. In the context of our current understanding of host defense against non-viral intracellular infections, we review recent data supporting a more significant role of B cells during M. tuberculosis infection than previously thought.

Synthesis, characterization, and immunogenicity in mice of Shigella sonnei O-specific oligosaccharide-core-protein conjugates

Shigellosis, an enteric disease, is on the World Health Organization's priority prevention list. In one study, the Shigella sonnei O-specific polysaccharide (O-SP)-protein conjugate showed 72% protection against disease in Israeli army recruits exposed to high rates (8-14%) of infection. The protection was related to vaccine-induced IgG anti-O-SP levels. Synthetic oligosaccharides of Shigella dysenteriae type 1, bound by their reducing ends to a carrier protein ("sun"-type configuration), induced significantly higher antibody levels than the native O-SP bound to protein by multiple-point attachments ("lattice"-type configuration). Attempts to synthesize the S. sonnei O-SP based oligosaccharides were not successful. Here, we describe the isolation, characterization, and conjugation of low-molecular-mass O-SP-core (O-SPC) fragments. The O-SPC fragments were bound by their reducing ends similar to the preparation of the synthetic S. dysenteriae type 1 conjugates. The O-SPC conjugates used oxime linkages between the terminal Kdo residues at the reducing ends of the S. sonnei saccharides and aminooxy linkers bound to BSA or a recombinant diphtheria toxin. The coupling reaction was carried out at a neutral pH and room temperature. IgG antibody levels induced in young outbred mice by the S. sonnei O-SPC conjugates were significantly higher then those elicited by the O-SP conjugates. Accordingly, we propose to evaluate clinically these conjugates.

Saccharides cross-reactive with Bacillus anthracis spore glycoprotein as an anthrax vaccine component

Bacillus anthracis is a spore-forming bacterium that causes anthrax in humans and in other mammals. The glycoprotein BclA (Bacillus collagen-like protein of anthracis) is a major constituent of the exosporium, the outermost surface of B. anthracis spores. The glycosyl part of BclA is an oligosaccharide composed of 2-O-methyl-4-(3-hydroxy-3-methylbutanamido)-4,6-dideoxy-d-glucose, referred to as anthrose, and three rhamnose residues. A structure similar to anthrose, 4-(3-hydroxy-3-methylbutanamido)-4,6-dideoxy-d-glucose is found in the side chain of the capsular polysaccharide (CPS) of Shewanella spp. MR-4. Under certain growth conditions the bacteria produce a variant CPS lacking one methyl group on the hydroxybutyrate, 4-(3-hydroxybutanamido)-4,6-dideoxy-d-glucose. Contrary to anthrose, neither of the Shewanella CPSs is 2-O methylated. Here, we report that both Shewanella CPS variants react with anti-B. anthracis spore sera. We also found that these antisera reacted with flagellae of Pseudomonas syringae, reported to be glycosylated with a similar terminal saccharide, 4-(3-hydroxybutanamido)-4,6-dideoxy-2-O-methyl-d-glucose. Sera produced by immunization with Shewanella or P. syringae cells bound to B. anthracis spores but not to Bacillus cereus spores in a fluorescent microscopy assay. These experiments show that methylation of the anthrose at the O-2 of the sugar ring and at the C-3 of 3-hydroxybutyrate are not essential for induction of cross-reactive antibodies. We report the preparation, characterization, and antibody responses to protein conjugates of the two variants of Shewanella CPS. Both conjugates induced antibodies that bound to both Shewanella CPS variants by ELISA and to B. anthracis spores, as detected by fluorescent microscopy. We propose the use of Shewanella CPS conjugates as a component of an anthrax vaccine.

Saccharide/protein conjugate vaccines for Bordetella species: preparation of saccharide, development of new conjugation procedures, and physico-chemical and immunological characterization of the conjugates

Bordetellae are Gram-negative bacilli causing respiratory tract infections of mammals and birds. Clinically important are B. pertussis, B. parapertussis and B. bronchiseptica. B. pertussis vaccines have been successful in preventing pertussis in infants and children. Veterinary vaccines against B. bronchiseptica are available, but their efficacy and mode of action are not established. There is no vaccine against B. parapertussis. Based on the concept that immunity to non-capsulated Gram-negative bacteria may be conferred by serum IgG anti-LPS we studied chemical, serological and immunological properties of the O-specific polysaccharides (O-SP) of B. bronchiseptica and B. parapertussis obtained by different degradation procedures. One type of the B. parapertussis and two types of B. bronchiseptica O-SP were recognized based on the structure of their non-reducing end saccharide; no cross-reaction between the two B. bronchiseptica types was observed. Competitive inhibition assays showed the immunodominance of the non-reducing end of these O-SP. Conjugates of B. bronchiseptica and B. parapertussis O-SP were prepared by two methods: using the anhydro-Kdo residue exposed by mild acid hydrolysis of the LPS or the 2,5-anhydromannose residue exposed by deamination of the core glucosamine of the LPS, for binding to an aminooxylated protein. Both coupling methods were carried out at a neutral pH, room temperature, and in a short time. All conjugates, injected as saline solutions at a fraction of an estimated human dose, induced antibodies in mice to the homologous O-SP. These methodologies can be applied to prepare O-SP-based vaccines against other Gram-negative bacteria.

Immunization with a Pseudomonas aeruginosa 1244 pilin provides O-antigen-specific protection

The O antigen is both a major structural outer membrane component and the dominant epitope of most gram-negative bacteria. Pseudomonas aeruginosa 1244 produces a type IV pilus and covalently links an O-antigen repeating unit to each pilin monomer. Here we show that immunization of mice with pure pilin from strain 1244 by use of either the mouse respiratory model or the thermal injury model resulted in protection from challenge with a pilus-null O-antigen-producing 1244 mutant. These results provide evidence that the pilin glycan stimulates a protective response that targets the O antigen, suggesting that this system could be used as the basis for the development of a variety of bioconjugate vaccines protective against gram-negative bacteria.

Synthesis and biological evaluation of phosphono analogues of capsular polysaccharide fragments from Neisseria meningitidis A

Neisseria meningitidis type A (MenA) is a Gram-negative encapsulated bacterium that may cause explosive epidemics of meningitis, especially in the sub-Saharan region of Africa. The development and manufacture of an efficient glycoconjugate vaccine against Neisseria meningitidis A is greatly hampered by the poor hydrolytic stability of its capsular polysaccharide, which is made up of (1-->6)-linked 2-acetamido-2-deoxy-alpha-D-mannopyranosyl phosphate repeating units. Since this chemical lability is a product of the inherent instability of the phosphodiester bridges, here we report the synthesis of phosphonoester-linked oligomers of N-acetyl mannosamine as candidates for stabilised analogues of the corresponding phosphate-bridged saccharides. The installation of each interglycosidic phosphonoester linkage was achieved by Mitsunobu coupling of a glycosyl C-phosphonate building block with the 6-OH moiety of a mannosaminyl residue. Each of the synthesised compounds contains an O-linked aminopropyl spacer at its reducing end (alpha- or beta-oriented) to allow for protein conjugation. The relative affinities of the synthetic molecules were investigated by a competitive ELISA assay and showed that a human polyclonal anti-MenA serum can recognise both the phosphonoester-bridged fragments 1-3 and their monomeric subunits, glycosides 20 and 21. Moreover, the biological results suggest that the abilities of these compounds to inhibit the binding of a specific antibody to MenA polysaccharide are dependent on the chain lengths of the molecules, but independent on the orientations of the anomeric linkers.

Effect of the nonreducing end of Shigella dysenteriae type 1 O-specific oligosaccharides on their immunogenicity as conjugates in mice

Endemic and epidemic shigellosis, an acute invasive disease of the lower intestines, afflicts millions of people worldwide with an estimated one million fatalities per annum at a low infectious dose. Our approach to vaccine development against Shigella is based on the hypothesis that serum IgG antibodies to the O-specific polysaccharide (O-SP) domains of the LPS of these organisms confer protection to infection. The synthetic oligosaccharides corresponding to the tetrasaccharide repeating unit of the O-SP of Shigella dysenteriae type 1 covalently linked to human serum albumin elicited O-SP-specific IgG in mice. The antibody levels were a function of both the saccharide chain length and their loading on the protein. These synthetic saccharide conjugates elicited significantly higher levels of IgG anti O-SP than conjugates prepared with the O-SP from the bacteria. Here, we evaluated the influence of the nonreducing terminal monosaccharide on the serum antibody response. To this end, we prepared synthetic oligosaccharides comprising hexa- to tridecasaccharide fragments of the native O-SP, having one of the four monosaccharide residues that constitute the repeating unit at their termini and bound them to BSA by a single-point attachment. The conjugates contained an average of 19 saccharide chains per BSA. The synthetic oligosaccharides inhibited the binding of serum raised against whole bacteria to its LPS to a similar extent but lower than the native O-SP. The highest anti-LPS levels were elicited by conjugates having N-acetylglucosamine (10-mer) or galactose residues (7- and 11-mers) at their nonreducing termini.

The Meningitis Vaccine Project

Epidemic meningococcal meningitis is an important public health problem in sub-Saharan Africa. Current control measures rely on reactive immunizations with polysaccharide (PS) vaccines that do not induce herd immunity and are of limited effectiveness in those under 2 years of age. Conversely, polysaccharide conjugate vaccines are effective in infants and have consistently shown an important effect on decreasing carriage, two characteristics that facilitate disease control. In 2001 the Meningitis Vaccine Project (MVP) was created as a partnership between PATH and the World Health Organization (WHO) with the goal of eliminating meningococcal epidemics in Africa through the development, licensure, introduction, and widespread use of conjugate meningococcal vaccines. Since group A Neisseria meningitidis (N. meningitidis) is the dominant pathogen causing epidemic meningitis in Africa MVP is developing an affordable (US$ 0.40 per dose) meningococcal A (Men A) conjugate vaccine through an innovative international partnership that saw transfer of a conjugation and fermentation technology to a developing country vaccine manufacturer. A Phase 1 study of the vaccine in India has shown that the product is safe and immunogenic. Phase 2 studies have begun in Africa, and a large demonstration study of the conjugate vaccine is envisioned for 2008-2009. After extensive consultations with African public health officials a vaccine introduction plan has been developed that includes introduction of the Men A conjugate vaccine into standard Expanded Programme on Immunization (EPI) schedules but also emphasizes mass vaccination of 1-29 years old to induce herd immunity, a strategy that has been shown to be highly effective when the meningococcal C (Men C) conjugate vaccine was introduced in several European countries. The MVP model is a clear example of the usefulness of a "push mechanism" to finance the development of a needed vaccine for the developing world.

Computer-assisted 2-D agarose electrophoresis ofHaemophilus influenzae type B meningitis vaccines and analysis of polydisperse particle populations in the size range of viruses: A review

When protein-polysaccharide conjugated vaccines were first developed for the immunization of small children against meningitis caused by infection with Haemophilus influenzae type b (Hib), the vaccine preparations varied in immunogenicity. Testing for immunogenicity was time-consuming and alternative analytical procedures for determining vaccine quality were unsatisfactory. For example, due to the very high molecular weight of the vaccine particles, immunogens could only be physically characterized as a fraction in the void volume of Sepharose gel filtration. In search of better analytical methods, a computer-assisted electrophoretic technique for analyzing such vaccines was developed in the period from 1983 to 1995. This new approach made it possible to analyze highly negatively charged particles as large as or larger than intact viruses. 2-D gel patterns were generated that varied depending on the conditions of the particular vaccine preparation and were therefore characteristic of each vaccine sample. Thus, vaccine particle populations with a continuous size variation over a wide range (polydisperse) could be characterized according to size and free mobility (related to particle surface net charge density). These advances are reviewed in this article, since the developed methods are still a promising tool for vaccine quality control and for predicting immunogen effectiveness in the production of vaccines. The technique is potentially beneficial for Hib immunogens and other high-molecular-mass vaccines. Additional biomedical applications for this nondenaturing electrophoretic technique are briefly discussed and detailed information about computational and mathematical procedures and theoretical aspects is provided in the Appendices.

Long-lasting and transmission-blocking activity of antibodies to Plasmodium falciparum elicited in mice by protein conjugates of Pfs25

Malaria is a leading cause of morbidity and mortality, estimated to cause >1 million childhood deaths annually. Plasmodium falciparum causes the most severe form of the disease. There is as yet no licensed vaccine for this disease, despite over a half century of research. In this study, we investigated a transmission-blocking vaccine candidate, the ookinete surface protein Pfs25. Antibodies against Pfs25, drawn in during a bite, can block parasite development in the mosquito midgut, preventing transmission to other individuals. Pfs25 is a low-molecular-weight protein, by itself not immunogenic. To increase its immunogenicity, we investigated several methods of conjugating Pfs25 to itself and to other proteins: recombinant Pseudomonas aeruginosa exotoxin A, and ovalbumin, using amide, hydrazone, or thioether linkages. All conjugates were immunogenic and induced booster responses in mice. The scheme to form amide bonds between proteins by using adipic acid dihydrizide as a linker produced the most immunogenic conjugates. Adsorption of the conjugates onto aluminum hydroxide further increased the antibody response. Remarkably, the antibody levels 3 or 7 months after the last injection were significantly higher than those 1 wk after that injection. The observed transmission-blocking activity of immune sera correlated with antibody levels measured by ELISA.

Sucres et vaccins : du polysaccharide purifié au glycoconjugué semi-synthétique

Over the last decades, capsular polysaccharides have been successfully used as antibacterial vaccines. Marketing several polysaccharide-protein conjugate vaccines filled the gap in many areas of children and infant vaccination. By facilitating access to structures of increasing complexity, recent progress in glycochemistry has enabled the design of more and more precisely defined glycoconjugate vaccines using synthetic saccharide components which mimic epitopes naturally implicated in protection. This strategy was recently validated in humans. It opens the way to new perspectives in vaccine research devoted to prophylactic and/or therapeutic applications against bacterial, fungal, parasitic or viral infections, and certain cancers.

Monitoring activation sites on polysaccharides by GC–MS

A method has been developed to determine the location and order of activation for potential saccharide antigens used in conjugate vaccine development. Saccharides were monitored for activation by sodium periodate oxidation and subsequent analysis by gas chromatography-mass spectrometry (GC-MS). Pneumococcal serotype polysaccharides 7F and 18C were evaluated as polysaccharides containing multiple potential sites for activation. Sialyllactose was used as a model oligosaccharide compound to evaluate oxidation of terminally linked sialic acids and reducing sugar residues. Oxidized saccharides were analyzed by monosaccharide composition and/or linkage analysis to elucidate specific activation of cis versus trans diols, as well as diols containing primary versus secondary alcohols, at specified levels of periodate. Samples (100-500 microg) were sequentially oxidized, reduced, methanolyzed, and derivatized in a single reaction vial for routine analysis by GC-MS.

Additional conjugation methods and immunogenicity of Bacillus anthracis poly-gamma-D-glutamic acid-protein conjugates

The capsule of Bacillus anthracis, composed of poly-gamma-d-glutamic acid (gammaDPGA), is an essential virulence factor of B. anthracis. The capsule inhibits innate host defense through its antiphagocytic action. gammaDPGA is a poor immunogen, but when covalently bound to a carrier protein, it elicits serum antibodies. To identify the optimal construct for clinical use, synthetic gammaDPGAs of different lengths were bound to carrier proteins at different densities. The advantages of the synthetic over the natural polypeptide are the homogeneous chain length and end groups, allowing conjugates to be accurately characterized and standardized and their chemical compositions to be related to their immunogenicities. In the present study, we evaluated, in addition to methods reported by us, hydrazone, oxime, and thioether linkages between gammaDPGA and several proteins, including bovine serum albumin, recombinant Pseudomonas aeruginosa exotoxin A, recombinant B. anthracis protective antigen (rPA), and tetanus toxoid (TT). The effects of the dosage and formulation on the immunogenicities of the conjugates were evaluated in mice. All conjugates were immunogenic. The optimal gammaDPGA chain length of 10 to 15 amino acids and the density, an average of 15 mol gammaDPGA per mol of protein, were confirmed. The thioether bond was the optimal linkage type, and TT and rPA were the best carriers. The optimal dosage was 1.2 to 2.5 microg of gammaDPGA per mouse, and adsorption of the conjugates onto aluminum hydroxide significantly increased the antibody response to the protein with a lesser effect on anti-gammaDPGA levels.

Recent developments in Neisseria meningitidis group A conjugate vaccines

Meningococcal disease, both endemic and epidemic, remains a major cause of meningitis in many countries. Protective immunity is mediated primarily by bacteriocidal antibodies against the capsular polysaccharides for serogroups other than B, and against non-capsular surface components for group B. This article focuses on the development of conjugate vaccines for serogroup A, with special emphasis on the needs of Africa. The first licensed (1999) meningococcal conjugate was against group C in the UK and was > 90% effective in infants, children and young adults. The problem now is to develop a highly immunogenic group A meningococcal conjugate vaccine for use in developing countries as an alternative to the presently licensed group AC polysaccharide vaccine. Immunogenicity studies on the group A polysaccharide show the polysaccharide itself to be uniquely immunogenic in young children compared with other polysaccharides, making comparative studies with a highly immunogenic conjugate of considerable importance.

Evaluation of a novel Vi conjugate vaccine in a murine model of salmonellosis

Immunisation of BALB/c mice with a vaccine containing Vi polysaccharide conjugated to the Klebsiella pneumoniae outer membrane 40 kDa protein (rP40), in combination with Escherichia coli heat-labile toxin adjuvant (LT), elicited anti-Vi IgG antibodies after administration using different routes. Testing of the immune serum in opsonisation assays demonstrated the specific enhancement of Vi-positive bacterial uptake by cultured murine bone marrow derived macrophages. Intra-peritoneal challenge of mice immunised with the Vi-based vaccine elicited a degree of protection against virulent Vi+ Salmonella enterica serovar typhimurium (S. typhimurium). In contrast, Vi vaccination did not confer protection against oral challenge with virulent Vi-positive S. typhimurium or S. dublin.

A practical protocol for carbohydrate microarrays

We have established a high-throughput biochip platform for constructing carbohydrate microarrays. Using this technology, carbohydrate-containing macromolecules of diverse structures, including polysaccharides, natural glycoconjugates, and mono- and oligosaccharides coupled to carrier molecules, can be stably immobilized on a glass chip without chemical modification. Here, we describe a practical protocol for this technology. We hope that anyone who has access to a standard cDNA microarray facility will be able to explore this technology for his or her own research interest. We also provide an example to illustrate that the carbohydrate microarray is also a discovery tool; this is particularly useful for identifying immunologic sugar moieties, including complex carbohydrates of cancer cells and sugar signatures of previously unrecognized microbial pathogens.

Future vaccine development at NICHD

Using published data and the results of our studies, we hypothesized that a critical level of serum IgG antibodies to the surface structures of invasive pathogens (capsular polysaccharides of Haemophilus influenzae type b, pneumococcus, meningococcus, Salmonella typhi, Escherichia coli, and Staphylococcus aureus, the O-specific polysaccharide LPS domain of the LPS of Shigella, non-typhoidal Salmonella, and E. coli, and the capsular polypeptide of Bacillus anthraces) confer immunity to these pathogens. Covalent attachment to a protein increases their immunogenicity and bestows T-cell properties to these antigens. We have also shown that a critical level of serum IgG antibodies to pertussis toxin alone induces immunity on both an individual and on a community basis (herd immunity) to Bordetella pertussis. It is likely that all the above conjugates and pertussis toxoid will be incorporated into vaccines for routine infant immunization.

Vaccines based on the cell surface carbohydrates of pathogenic bacteria

Glycoconjugate vaccines, in which a cell surface carbohydrate from a micro-organism is covalently attached to an appropriate carrier protein are proving to be the most effective means to generate protective immune responses to prevent a wide range of diseases. The technology appears to be generic and applicable to a wide range of pathogens, as long as antibodies against surface carbohydrates help protect against infection. Three such vaccines, against Haemophilus influenzae type b, Neisseria meningitidis Group C and seven serotypes of Streptococcus pneumoniae, have already been licensed and many others are in development. This article discusses the rationale for the development and use of glycoconjugate vaccines, the mechanisms by which they elicit T cell-dependent immune responses and the implications of this for vaccine development, the role of physicochemical methods in the characterisation and quality control of these vaccines, and the novel products which are under development.

Les vaccins conjugués

Encapsulated bacterial pathogens (e.g. Haemophilus influenzae type b [Hib], Neisseria meningitidis, or Streptococcus pneumoniae) target infants and young children who have lost any protective anti-capsular antibodies supplied maternally and whose immune systems are ineffective against T-independent antigens such as the polysaccharides of the capsule. The polysaccharide-protein conjugate vaccines overcome this limitation by converting the polysaccharide to a T-dependent antigen, which allows a vaccinated infant to mount a protective immune response. Where conjugated vaccines have been introduced into paediatric vaccination schedules, the incidence of invasive diseases caused by Hib, the group C meningococcus, or the pneumococcus has plummeted by at least 80%, a major public health success. Furthermore, surveillance has demonstrated that the conjugate vaccines provide 'herd protection' through their beneficial impact on nasopharyngeal colonisation among vaccinated children. Promising future approaches include enhancement of the number of capsular serogroups targeted by the meningococcal or pneumococcal conjugate vaccines.

Vaccines: past, present and future

The vaccines developed over the first two hundred years since Jenner's lifetime have accomplished striking reductions of infection and disease wherever applied. Pasteur's early approaches to vaccine development, attenuation and inactivation, are even now the two poles of vaccine technology. Today, purification of microbial elements, genetic engineering and improved knowledge of immune protection allow direct creation of attenuated mutants, expression of vaccine proteins in live vectors, purification and even synthesis of microbial antigens, and induction of a variety of immune responses through manipulation of DNA, RNA, proteins and polysaccharides. Both noninfectious and infectious diseases are now within the realm of vaccinology. The profusion of new vaccines enables new populations to be targeted for vaccination, and requires the development of routes of administration additional to injection. With all this come new problems in the production, regulation and distribution of vaccines.

Haemophilus influenzae type b conjugate vaccines

Haemophilus influenzae type b (Hib) is one of the leading causes of invasive bacterial infection in young children worldwide. During childhood, acquisition of antibody directed against the polysaccharide capsule of the organism, presumably as a result of asymptomatic carriage, confers protection and disease is much less common after the age of 4 years. Like other polysaccharides, the polyribosyl ribitol phosphate (PRP) of the Hib capsule is a T-independent antigen and not immunogenic when administered as a vaccine in infancy. Because the highest rates of disease occur in the first 2 years of life, efficacious Hib vaccines have been designed by covalently linking the PRP capsule to a carrier protein that recruits T-cell help for the polysaccharide immune response and induces anti-PRP antibody production even in the first 6 months of life. Introduction of Hib protein-polysaccharide conjugate vaccines into many industrialized countries over the past 15 years has resulted in the virtual elimination of invasive Hib disease. However, despite the success of the vaccine programme several factors may interfere with the effectiveness of the vaccine in the routine programme, as observed in the UK recently. Such factors may include interference with other concomitant vaccines, waning immunity in the absence of booster doses of vaccine, and reduced natural boosting as a result of decreased transmission of the organism. However, the burden of disease remains highest in resource-poor countries and urgent efforts are needed to provide the benefits of this vaccine for children living in regions where it cannot be used for economic and logistical reasons.

Chemical structure, conjugation, and cross-reactivity of Bacillus pumilus Sh18 cell wall polysaccharide

Bacillus pumilus strain Sh18 cell wall polysaccharide (CWP), cross-reactive with the capsular polysaccharide of Haemophilus influenzae type b, was purified and its chemical structure was elucidated using fast atom bombardment mass spectrometry, nuclear magnetic resonance techniques, and sugar-specific degradation procedures. Two major structures, 1,5-poly(ribitol phosphate) and 1,3-poly(glycerol phosphate), with the latter partially substituted by 2-acetamido-2-deoxy-alpha-galactopyranose (13%) and 2-acetamido-2-deoxy-alpha-glucopyranose (6%) on position O-2, were found. A minor component was established to be a polymer of -->3-O-(2-acetamido-2-deoxy-beta-glucopyranosyl)-1-->4-ribitol-1-OPO3-->. The ratios of the three components were 56, 34, and 10 mol%, respectively. The Sh18 CWP was covalently bound to carrier proteins, and the immunogenicity of the resulting conjugates was evaluated in mice. Two methods of conjugation were compared: (i) binding of 1-cyano-4-dimethylaminopyridinium tetrafluoroborate-activated hydroxyl groups of the CWP to adipic acid dihydrazide (ADH)-derivatized protein, and (ii) binding of the carbodiimide-activated terminal phosphate group of the CWP to ADH-derivatized protein. The conjugate-induced antibodies reacted in an enzyme-linked immunosorbent assay with the homologous polysaccharide and with a number of other bacterial polysaccharides containing ribitol and glycerol phosphates, including H. influenzae types a and b and strains of Staphylococcus aureus and Staphylococcus epidermidis.

Humoral immune responses to a protective peptide-conjugate against measles after different prime-boost regimens

The current live-attenuated measles vaccine leaves many children unprotected until they reach the recommended age of vaccination. We have previously shown that the short peptide corresponding to the hemagglutinin noose epitope (HNE) of the measles virus (MV) hemagglutinin protein induced virus-neutralizing antibodies even in the presence of protective levels of anti-whole virus-specific antibodies. Here we investigate the immunogenicity of HNE peptide-conjugates of diphtheria or tetanus toxoid in mice after active and passive priming with antibodies against the peptide, toxoids and conjugates. Both conjugates induced high titers of peptide antibodies which crossreacted with the virus and protected against a lethal intracranial challenge with a rodent-adapted measles virus, even after active priming with homologous or heterologous toxoid or conjugate. Peptide-specific epitopic suppression was stronger after passive priming with carrier or conjugate antibodies, but diphtheria toxoid as a carrier was less susceptible to suppression than tetanus toxoid and suppression was overcome by an additional boost. Furthermore, prior immunization with peptide-conjugate did not interfere with the development of a complete response to a subsequent injection of MV, suggesting that the benefits of a follow-up vaccination with the current live-attenuated vaccine would not be lost. These results underline the potential of these peptide-based conjugates as vaccine candidates for use in early infancy to close the window of susceptibility before the live-attenuated vaccine can be administered.

A Synthetic Conjugate Polysaccharide Vaccine Against Haemophilus influenzae Type b

Glycoconjugate vaccines provide effective prophylaxis against bacterial infections. To date, however, no commercial vaccine has been available in which the key carbohydrate antigens are produced synthetically. We describe the large-scale synthesis, pharmaceutical development, and clinical evaluation of a conjugate vaccine composed of a synthetic capsular polysaccharide antigen of Haemophilus influenzae type b (Hib). The vaccine was evaluated in clinical trials in Cuba and showed long-term protective antibody titers that compared favorably to licensed products prepared with the Hib polysaccharide extracted from bacteria. This demonstrates that access to synthetic complex carbohydrate-based vaccines is feasible and provides a basis for further development of similar approaches for other human pathogens.

Development of experimental carbohydrate-conjugate vaccines composed of Streptococcus pneumoniae capsular polysaccharides and the universal helper T-lymphocyte epitope (PADRE®)

Experimental carbohydrate-conjugate vaccines composed of the 13 amino acid universal Pan HLA-DR Epitope (PADRE) and Streptococcus pneumoniae capsular polysaccharides from serotypes 14, 6B and 9V were produced. Simple carbodiimide-mediated condensation chemistry was used to conjugate the PADRE synthetic peptide to the three chemically different capsular polysaccharides in a 1:1 molar ratio. The immunogenicity of the PADRE peptide component of the conjugate vaccines was confirmed by the induction of PADRE-specific CD4+ helper T cell (HTL) responses following immunization of C57BL/6 mice. High titer antibody responses specific for polysaccharides of S. pneumoniae serotypes 14, 6B and 9V were induced using Complete Freund's Adjuvant (CFA) and alhydrogel Al(OH)3 formulations. The HTL, or carrier, effect of the PADRE synthetic peptide was only evident using the PADRE-polysaccharide conjugates; simple mixtures of the PADRE peptide and polysaccharides were essentially nonimmunogenic. The functional or potential protective value of the polysaccharide-specific antibodies was measured as a function of opsonophagocytic activity for the 6B serotype. High titers of opsonophagocytic activity were measured in sera from mice immunized with formulations containing both adjuvants. These data demonstrate that the PADRE synthetic peptide can induce the HTL responses needed to support the development of antibodies specific for bacterial carbohydrates used in conjugate vaccines.

Detection of O-acetylated Vi polysaccharide of Salmonella enterica subspecies typhi by Enzyme Immunoassay

Immunisation with capsular Vi polysaccharide (Vi PS) of Salmonella enterica serovar Typhi (S. typhi) protects against typhoid. This protection depends on the presence of O-acetyl groups on the Vi PS, which form an immunodominant epitope. An antiserum raised against conjugated Vi PS was used as the basis for an indirect Enzyme Immunoassay (EIA). The antiserum did not react with lipopolysaccharide of five gram negative bacteria including S. typhi. Vi PS from three different sources was tested, and all but one of 18 native Vi PS preparations had EIA values comparable to a standard Vi PS preparation. The sensitivity of the EIA for the detection of O-acetyl groups on Vi PS was compared to an NMR spectroscopy assay (Biologicals 28 (2000) 17-24). The EIA distinguished between O-acetylated and de-O-acetylated Vi PS preparations. However, significantly lower EIA reactivity was observed only for samples which had O-acetylation levels of 25% or less. This assay should facilitate batch control of Vi vaccines.

Preparation and characterization of group A meningococcal capsular polysaccharide conjugates and evaluation of their immunogenicity in mice

Epidemic and endemic meningitis caused by group A Neisseria meningitidis remains a problem in sub-Saharan Africa. Although group A meningococcal capsular polysaccharide (GAMP) vaccine confers immunity at all ages, the improved immunogenicity of a conjugate and its compatibility with the World Health Organization's Extended Program on Immunization offers advantages over GAMP alone. Conjugates of GAMP bound to bovine serum albumin (BSA) were synthesized, characterized, and evaluated for their immunogenicities in mice. Two methods, involving adipic acid dihydrazide (ADH) as a linker, were used. First, ADH was bound to GAMP activated with cyanogen bromide (CNBr) or with 1-cyano-4(dimethylamino)-pyridinium tetrafluoroborate (CDAP) to form GAMP(CNBr)AH and GAMP(CDAP)AH. These derivatives were bound to BSA by 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide (EDC) to form GAMP(CNBr)AH-BSA and GAMP(CDAP)AH-BSA. Second, ADH was bound to BSA with EDC to form AHBSA. AHBSA was bound to activated GAMP to form GAMP(CNBr)-AHBSA and GAMP(CDAP)-AHBSA. The yield of GAMP(CDAP)-AHBSA (35 to 40%) was higher than those of the other conjugates (5 to 20%). GAMP conjugates elicited immunoglobulin G (IgG) anti-GAMP in all mice after three injections of 2.5 or 5.0 microg of GAMP: the geometric mean (GM) was highest in recipients of GAMP(CDAP)-AHBSA (11.40 enzyme-linked immunosorbent assay units). Although the difference was not statistically significant, the 5.0- microg dose elicited a higher GM IgG anti-GAMP than the 2.5- microg dose. Low levels of anti-GAMP were elicited by GAMP alone. GAMP(CDAP)-AHBSA elicited bactericidal activity roughly proportional to the level of IgG anti-GAMP.

Poly(gamma-D-glutamic acid) protein conjugates induce IgG antibodies in mice to the capsule of Bacillus anthracis: a potential addition to the anthrax vaccine

Both the protective antigen (PA) and the poly(gamma-d-glutamic acid) capsule (gamma dPGA) are essential for the virulence of Bacillus anthracis. A critical level of vaccine-induced IgG anti-PA confers immunity to anthrax, but there is no information about the protective action of IgG anti-gamma dPGA. Because the number of spores presented by bioterrorists might be greater than encountered in nature, we sought to induce capsular antibodies to expand the immunity conferred by available anthrax vaccines. The nonimmunogenic gamma dPGA or corresponding synthetic peptides were bound to BSA, recombinant B. anthracis PA (rPA), or recombinant Pseudomonas aeruginosa exotoxin A (rEPA). To identify the optimal construct, conjugates of B. anthracis gamma dPGA, Bacillus pumilus gamma dLPGA, and peptides of varying lengths (5-, 10-, or 20-mers), of the d or l configuration with active groups at the N or C termini, were bound at 5-32 mol per protein. The conjugates were characterized by physico-chemical and immunological assays, including GLC-MS and matrix-assisted laser desorption ionization time-of-flight spectrometry, and immunogenicity in 5- to 6-week-old mice. IgG anti-gamma dPGA and antiprotein were measured by ELISA. The highest levels of IgG anti-gamma dPGA were elicited by decamers of gamma dPGA at 10 -20 mol per protein bound to the N- or C-terminal end. High IgG anti-gamma dPGA levels were elicited by two injections of 2.5 microg of gamma dPGA per mouse, whereas three injections were needed to achieve high levels of protein antibodies. rPA was the most effective carrier. Anti-gamma dPGA induced opsonophagocytic killing of B. anthracis tox-, cap+. gamma dPGA conjugates may enhance the protection conferred by PA alone. gamma dPGA-rPA conjugates induced both anti-PA and anti-gamma dPGA.

Levels of antibodies specific to tetanus toxoid, Haemophilus influenzae type b, and pneumococcal capsular polysaccharide in healthy children and adults

Antibody levels specific for capsular polysaccharides of Streptococcus pneumoniae and Haemophilus influenzae type b (Hib) and for tetanus toxoid were measured in serum samples of 386 age-stratified subjects. The study group consists of healthy adult blood donors and hospitalized children undergoing elective surgery, excluding individuals with a history of infection. In children, anti-tetanus toxoid antibody levels displayed two peaks of 1.20 IU/ml (20.4 mg/liter) and 1.65 IU/ml (28.1 mg/liter) related to the schedule of routine childhood immunization in the first year and at 8 years of age. Eighty percent of the antibodies are of the immunoglobulin G1 (IgG1) isotype. For pneumococcal capsular polysaccharide (PCP), the specific antibody levels represent the acquisition of natural immunity. The initial concentration of 9.2 mg/liter was low in infancy (0.5 to 1 years of age) and remained low until 3 to 4 years of age (14.6 mg/liter). During this period PCP antibodies were almost 100% of the IgG2 subclass. Thereafter, IgG anti-PCP antibody titers increased steadily to adult levels (59.5 mg/liter). The data are intended to provide reference ranges to aid in the interpretation of specific antibody determinations in the clinical setting.

Pneumococcal conjugate vaccine (Prevnar; PNCRM7): a review of its use in the prevention of Streptococcus pneumoniae infection

PNCRM7 (Prevnar) is a pneumococcal vaccine containing seven capsular polysaccharide antigens from the bacterium Streptococcus pneumoniae, each of which is conjugated to diphtheria protein [cross-reactive material (CRM(197))]. CRM(197) is an inert but immunogenic variant of diphtheria toxoid that is also used as a carrier molecule in one Haemophilus influenzae type b conjugate vaccine. Unlike the 23-valent unconjugated pneumococcal vaccines, PNCRM7 elicits a T cell-dependent response and thus protects young children against pneumococcal disease. The immunogenicity of PNCRM7 has been demonstrated in both healthy children aged <2 years and older children in high-risk groups. Two randomized, double-blind trials conducted in the US demonstrated that all PNCRM7 serotypes were immunogenic in healthy infants and young children when compared with a control vaccine. A booster dose of PNCRM7 elicited an anamnestic response to all seven serotypes. Data from a large, randomized, double-blind study conducted in California (US) have confirmed the protective efficacy of PNCRM7 against invasive pneumococcal disease (e.g. bacteremia, meningitis) caused by serotypes included in the vaccine. The vaccine efficacy in the per-protocol analysis was 97.4% and its efficacy against invasive disease caused by any pneumococcal serotype in the intent-to-treat (ITT) analysis was 89.1%. Indeed, a postlicense surveillance study (n = 211,565) showed that the introduction and routine use of PNCRM7 was associated with a marked reduction in invasive pneumococcal disease in children <5 years of age. In addition, the US trial and another randomized, double-blind trial conducted in Finland, showed that PNCRM7 vaccine efficacy against all otitis media episodes was between 6 and 7%. PNCRM7 vaccine was generally well tolerated and had a similar local and systemic adverse events profile to other pediatric vaccines. The most common local adverse event associated with PNCRM7 administration was inflammation at the injection site, and the most common systemic adverse effect was febrile illness (> or =38 degrees C) that usually resolved without treatment. The limited available pharmacoeconomic data suggest that PNCRM7 could be cost effective depending, in part, on the manufacturer's list price of the vaccine. Results of the base case analysis in a US study showed a cost-effectiveness ratio for PNCRM7 of US dollars 80,000 per life-year saved from a societal perspective compared with US dollars 176,000 from a healthcare payer perspective, assuming a nondiscounted list price of US dollars 58 per dose (1997 costs). Concomitant administration of PNCRM7 vaccine with hepatitis B, oral polio, meningococcal oligosaccharide protein conjugate or H. influenzae type b vaccines did not affect the immunogenicity of these pediatric vaccines to a clinically relevant extent.

CONCLUSION

PNCRM7 vaccine will be of great benefit to those societies that have active immunization programs implemented. In infants and vulnerable children throughout the world, PNCRM7 vaccine has the potential to reduce the mortality and morbidity rates associated with S. pneumoniae infections. In developed countries, the vaccine will be of particular benefit in preventing disabling infections but its impact in developing countries will be more pronounced with the potential to greatly reduce mortality.

Donor immunization with pneumococcal conjugate vaccine and early protective antibody responses following allogeneic hematopoietic cell transplantation

Patients undergoing hematopoietic cell transplantation (HCT) are at increased risk for infections with Streptococcus pneumoniae and have long-lasting, impaired antibody responses to pneumococcal polysaccharide vaccines. We examined whether donor immunization with a heptavalent pneumococcal conjugate vaccine (PCV7) would elicit protective antibody responses to additional doses of vaccine administered early after transplantation. Ninety-six patients scheduled to receive an allogeneic hematopoietic cell transplant were randomized with their donors to receive either a dose of PCV7 vaccine or no vaccine before transplantation. All patients received PCV7 at 3 months, 6 months, and 12 months following transplantation, and serotype-specific antibody concentrations were determined after each dose. Following HCT, geometric mean antibody concentrations of patients in the immunized donor group were significantly higher for 5 of the 7 vaccine serotypes after one dose (P <.05) and for 4 of the 7 serotypes after 2 doses of vaccine (P <.03). Sixty-seven percent of patients in the immunized donor group had presumed protective IgG concentrations more than or equal to 0.50 microg/mL to all 7 serotypes following the first dose of vaccine compared to 36% in the unimmunized donor group (P =.05). After the third dose of vaccine, both groups had more than 60% of patients with concentrations at least 0.50 microg/mL to all vaccine serotypes. Donor immunization enhances early antibody responses of patients undergoing HCT to pneumococcal conjugate vaccine. A 3-dose schedule of PCV7 vaccine at 3, 6, and 12 months is immunogenic in these patients regardless of donor immunization.

Haemophilus influenzae type b vaccine formulation and risk of childhood leukaemia

Incidence of childhood leukaemia was studied among subjects of a vaccine trial in Finland comparing the polysaccharide-diptheria toxoid conjugate and oligosaccharide-CRM197 conjugate Haemophilus influenzae type b conjugate vaccine formulations. Eighty cases of childhood leukaemia were detected: 35 among children on the polysaccharide-diptheria toxoid conjugate arm, and 45 among children on the oligosaccharide-CRM197 conjugate arm, which was not statistically significant.

Streptococcus pneumoniae polysaccharides conjugated to the outer membrane protein A from Klebsiella pneumoniae elicit protective antibodies

Polysaccharides (PSs) derived from Streptococcus pneumoniae include more than 90 serotypes and differ greatly in their immunogenicity. In addition, immunization with PSs does not induce high affinity antibody production and no memory B-cells are generated. Coupling PSs to carrier proteins has been reported to induce B-cell maturation and to install a B-cell memory. As an alternative carrier protein, the outer membrane protein A (OmpA) derived from Klebsiella pneumoniae has been coupled to various PSs. We evaluated the immunogenicity of two PS conjugates, using PS derived from S. pneumoniae types 14 and 19. In this report, we show that anti-PS IgG responses are generated after the conjugation of PSs to P40. In addition, the humoral response generated is able to protect mice from a bacterial challenge. Our results indicate that P40 could be included in the development of new PS conjugate vaccines.

Immunogenicity of anti-Haemophilus influenzae type b CRM197 conjugate following mucosal vaccination with oligodeoxynucleotide containing immunostimulatory sequences as adjuvant

Most vaccines are delivered by injection. Mucosal vaccination would increase compliance and decrease the risk of spread of infectious diseases due to a reduction of mucosal colonization and of contaminated syringes. However, most vaccines are unable to induce immune responses when administered mucosally, and require the use of strong adjuvant or effective delivery systems. Synthetic oligodeoxynucleotides (ODN) containing CpG immunostimulatory sequences (ISS) have been shown to act as potent adjuvants of type-1 immune responses also when mucosally co-administered with protein or peptide vaccines. We have shown that ISS can increase the anti-polysaccharide polyribosyl ribitol phosphate (PRP) antibody titres and anti-diphtheria toxin neutralizing antibody, if used as adjuvant of anti-Haemophilus influenzae type b (Hib) PRP vaccine conjugated with cross-reacting material (CRM) of diphtheria toxin in mice. Here, we show that ISS have the potential to increase host local and systemic antibody response against both the PRP and the protein component of a conjugated vaccine when mucosally administered in mice. Mucosal administration of Hib-CRM vaccine induced anti-PRP and neutralizing anti-diphtheria toxin antibodies of all the IgG subclasses, with a predominance of type-1 immune response-associated IgG2a and IgG3. At odds with systemic administration, the mucosal delivery of Hib-CRM induced anti-PRP and anti-diphtheria toxin mucosal IgA. These data envisage the feasibility of a mucosal vaccination with an already licensed Hib-CRM vaccine to achieve both an anti-H. influenzae and -diphtheria effective protection.

Low baseline antibody level to diphtheria is associated with poor response to conjugated pneumococcal vaccine in adults

Joining polysaccharide antigens to protein increases immunogenicity in infants. In older adults, using conjugation to protein carriers to improve the immune response to pneumococcal polysaccharide vaccine has thus far proved disappointing. Low immunity to the carrier protein in the elderly may explain the failure of conjugated vaccines to elicit a T-cell-dependent response. We immunized 49 older adults (ages 60-78) and 50 younger adults (ages 18-45) with either 23-valent pneumococcal polysaccharide (PS) vaccine or 5-valent CRM197-conjugated pneumococcal oligosaccharide. Sera obtained before and after vaccination were analyzed for antibody to pneumococcal serotypes 14 and 6B and diphtheria toxin by ELISA. Baseline diphtheria toxin antibody level was lower in older adults than in younger adults (0.31 and 0.88 IU/ml, respectively; p < 0.0001). Adults with higher diphtheria antibody level had a higher antibody level to PS type 6B after vaccination than those with lower diphtheria antibody level (9.9 vs. 3.5 microg/ml, respectively; p = 0.01). Antibody level to PS type 14 was higher, but differed by baseline anti-diphtheria antibody level only when the older group was evaluated alone. Low levels of antibody to diphtheria protein may explain some of the lower responses to conjugate pneumococcal vaccine in older adults.

Pneumococcal conjugate vaccines overcome splenic dependency of antibody response to pneumococcal polysaccharides

Protection against infections with Streptococcus pneumoniae depends on the presence of antibodies against capsular polysaccharides that facilitate phagocytosis. Asplenic patients are at increased risk for pneumococcal infections, since both phagocytosis and the initiation of the antibody response to polysaccharides take place in the spleen. Therefore, vaccination with pneumococcal polysaccharide vaccines is recommended prior to splenectomy, which, as in the case of trauma, is not always feasible. We show that in rats, vaccination with a pneumococcal conjugate vaccine can induce good antibody responses even after splenectomy, particularly after a second dose. The spleen remains necessary for a fast, primary response to (blood-borne) polysaccharides, even when they are presented in a conjugated form. Coadministration of a conjugate vaccine with additional nonconjugated polysaccharides of other serotypes did not improve the response to the nonconjugated polysaccharides. We conclude that pneumococcal conjugate vaccines can be of value in protecting asplenic or hyposplenic patients against pneumococcal infections.